A large number of electrophotographic methods are known and the electrophotographic method generally comprises a step of forming an electrical latent image on a photoreceptor by various means utilizing a photoconductive substance (exposure step), a development step of developing the latent image using a toner, a transfer step of transferring the toner image on a transferee material such as paper, and a step of fixing the toner image under heating, pressure or the like using a fixing roller.
The toner for use in such an electrophotographic method is produced by a pulverizing method, a polymerization method or a spray dry method.
The pulverizing method is a method of kneading a raw material containing a resin as a main component (hereinafter sometimes simply referred to as a “resin”) and a coloring agent at a temperature higher than the softening point of resin to obtain a kneaded material and then cooling and pulverizing the kneaded material. This pulverizing method is advantageous in that the raw material can be selected over a wide range and a toner can be relatively easily produced. However, the toner obtained by the pulverizing method varies widely in the shape among particles and the particle size distribution is disadvantageously liable to be broad. As a result, the electrical charging property, fixing property and the like vary widely among toner particles and the toner as a whole decreases in the reliability.
The polymerization method is a method of performing a polymerization reaction using a monomer as a constituent component of a resin in a liquid phase or the like to produce the objective resin and thereby produce a toner particle. This polymerization method is advantageous in that the toner particle obtained can have a shape relatively high in the sphericity (a shape close to a geometrically complete sphere). However, in the polymerization method, the fluctuation in the particle size among particles cannot be sufficiently reduced in some cases. Furthermore, in the polymerization method, the latitude in the selection of a resin material is narrow and a toner having objective properties is sometimes not obtained.
The spray dry method is a method where a raw material for the production of a toner, which is dissolved in a solvent, is sprayed using a high-pressure gas and thereby, a fine powder is obtained as a toner. The spray dry method is advantageous in that the above-described pulverizing step is not necessary. However, in this spray dry method, the raw material is sprayed using a high-pressure gas and therefore, the spraying conditions of the raw material cannot be precisely controlled, as a result, a toner particle having objective shape and size is difficult to produce with good efficiency. Furthermore, in the spray dry method, the particle size varies widely among particles formed by spraying and therefore, the moving speed also varies widely among particles. This causes collision or aggregation of sprayed particles before the sprayed raw material is solidified, and a powder of anomaly shapes is formed, as a result, the fluctuation in the shape and size sometimes more increases among finally obtained toner particles. As such, the toner obtained by the spray dry method varies widely in the shape and size among particles, therefore, the electrical charging property, fixing property and the like also vary widely among toner particles and the toner as a whole decreases in the reliability.